Acoustic wave filter



1943a w. P. MASON 2,308,886

ACOUSTIC WAVE FILTER I Filed Nov. 29, 1940 Q INVENTOR m R MASON ATTORNEYPatented Jan. 19, 1943 ACOUSTIC WAVE FILTER- Warren P. Mason, WestOrange, N. 1., assimor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application November 29, 1940,Serial No. 367,737

18 Claims.

This invention relates to acoustic wave filters and more particularly tofilters of the suppres sion type adapted to handle a large volume ofall.

The principal object of the invention is to suppress a band ofobjectionable frequencies while permitting the free how of a largevolume of all".

An air conditioning installation usually iii-- cludes a blower toprovide a forced circulation of air in the room to be conditioned. Theblowor may be arranged either to force air intothe room or to withdrawair from the room. In either case it is desirable to introduce anocoustic wave filter between the blower and the air duct communicatingwith the room in order to suppress the objectionable audible frequenciesset up by the blower. If a large room is being con ditioned the filtermust be capable of effectively suppressing the noise while permittingthe substontially unrestricted flow of a large volume of air per unit oftime.

The acoustic wave filter of the present invention is a. simple andefllcient device suitable for the purpose indicated. The filtercomprises o main conduit, preferably of rectangular crosssection, andone or more pairs of side branches arranged to form a symmetricalstructure about a central longitudinal plane. For proper funotioning ofthe filter only plane waves the ireenemies to be suppressed arepermitted to reach the openings into the side branches. 'Io accomplishthis the width dimension of the main com duit is limited to awave-length M corresponding to the highest frequency f0 to be suppressedin the sections of the filter with which the side branches areassociated. For the same reason the main conduit is divided in thedirection of its depth by one or more partitions which are parallel tothe width dimension and are spaced apart by a distance not exceeding in.These par titions are preferably extended into and all the way acrossthe side branches.

If the air entering the filter has an unsymmetrical particle velocity,as may be the case when the filter is fed by a blower, thecross-partitions at the input end of the main conduit preferably have aspacing which does not exceed a. halt that is, io. The half-wsve spac sobe advantageously e resin con Cit of each pair is omitted, thecross-partitions should have the halt-wave spacing throughout the entirelength of the main conduit, and also, preferably, in the side branches.

The different pairs of side branches preferably have diirerent lengthsso that they will hove diflferent frequencies of resonance. Theseresonances determine the frequencies 0! maximum attenuation for thefilter sections and they are so chosen that they all fall below In andare spaced'for maximum sustained attenuation in the band to besuppressed.

The spacing between the successive openings into the side branches andthe longitudinal dimensions of the side branches determine the lowercut-off of the filter, that is, the lowest Irequency which will beattenuated. For the most efllcient use of space the side branches occupythe full length of the main conduit, that is, a preceding branch abutsagainst the next succeeding one.

All of the partitions in the filter parallel to the direction of the airflow are preferably made rounded on the ends toward the flow and sharpon the ends from which the air leaves. For some of the partitions,especially those serving as guides in curved sections of the mainconduit, the strecmlimng may advantageously be carried further. For mostefficient results the openings into the side branches preferablyconstitute approximately one-third of the cross-sectional area of theside branch. To improve the suppression of undesired frequencies higherthan fo the main conduit may be extended at one end and lined with soundabsorbing material such, for example, as felt or glass wool. To providean impedance match between the filter and its load the discharge end ofthe main conduit may be flared by curving one or both pairs of sidesoutwardly.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdraw" ings, in which like reference characters represent like or similarparts and in which:

Fig. 1 is an elevation, partly in section, of an acoustic wave filter inaccordance with the invention;

Fig. 21s a plan view of the filter c! Fig. 1, part ly in section, alongthe line 2 2: and

Fig. 3 is a sectional view along the line 3-4 l, with parts of the sidebranches broken 2 nd i the a main conduit 4 or rectangular cross-sectionextending from one end to the other and a number of pairs of sidebranches 5, 5', 6, 6' and I, 1. The two branches of each pair arearranged opposite to each other to form a balanced structure. The sidebranches are closed chambers communicating with the main conduit 4 bymeans of openings such as 8 and 8'.

At the intake end l 0! the filter the main conduit I is extended in theform of a curved section i I which describes a 90 degree angle. Thesection H has a number of partitions l2, l3 and H which extend from oneside l to the other side I 8 and are parallel to the curved sides 29 and30.

At the discharge end H the main conduit 4 is extended in a section I!lined internally with a layer of sound absorbing material I! to increasethe suppression oi the higher frequencies. The section l8 terminates inan end portion 20 which has flaring sides 2| and 22 designed to matchthe impedance of the filter to that of the load. The partition 13 isextended through the main conduit 4, including the section l8, and allthe way across the side branches 5, 5', 5, 6', 1 and 1. Each side of thepartition I3 within the section i8 is also covered with a layer of soundabsorbing material i9. The layer l9 may, for example, be felt, glasswool or other suitable material and may have a thickness of perhaps oneinch. The walls and partitions of the filter may be made of any suitablemetal and may be welded, soldered or otherwise secured at the joints.

Some design considerations to be observed in dimensioning the filterwill now be taken up. It is known that the side branches of an acousticiilter will function properly only when a plane wave alone is propagateddown the main conduit. The conditions necessary to insure only a planewave in a conducting tube of rectangular cross-section will now beconsidered. The first case of interest is the one in which the air hasan unsymmetrical particle velocity.

The equation of motion for any type of sinusoidal air waves passingthrough a conduit of rectangular crcss-section is -o e Et 531 where e isthe velocity potential, to is 21 times,

=0 when z= in.

For the next simplest case, when m l and 12:0.

In Equation 6 it is apparent that ii the value of the expression undereach of the radicals is negative and therefore the square root isimaginary. This represents a wave which is attenuated as it ispropagated through the conduit. The other values of m and 11, representmore complicated wave forms which will be atw 1' fat tenuated to ahigher frequency in the conduit.

In expression. (7) if yr, the width of the conduit in the y direction,is substituted for 2b and 2140 is substituted for u: there is obtainedwhere M is the wave-length corresponding to the frequency in. The ruleto be deduced from expression (8) is that, for the unsymmetrical case,if the width 111 of the main conduit is kept less than then all waves offrequency Io or lower permitted to pass through the conduit will beplane waves. Since plane waves only are properly attenuated by the sidebranches of the filter it follows that In is the highest frequency whichcan be successfully suppressed by the filter in the sections includingthe side branches.

An unsymmetrical case would be presented if half of the side branches,for example branches 5, 6 and '7, were omitted. In this case thedimension yr should be half a wave-length. Another unsymmetrical caseoccurs where the direction of the air flow is changed, as in section 11.Here the partitions i2, i3 and H are spaced apart a distance at equal to/2)).

The symmetrical case will now be considered. The filter as shownrepresents such a case, since the side branches occur in oppositelydisposed pairs. The half-wave spacing of the partitions i2, I3 and I4assures only plane waves leaving the section II. The side branches,however, set up particle velocities in the 1/ direction, for thepressure wave causes air to flow into the side branches. However, due tothe balanced construction of the side branches, the direction of flowhas a plane of symmetry down the center and therefore any parasitic waveset up by the side branch must have an opposite flow on the two sides.

. waves involving 171:1 will have a particle 51:. 1. direction given bythat this wave will be attenuated rapidly as it passes along the mainbranch provided the dimension m is less than a wave-length for thefrequency Io, that is,

A comparison of expressions '(8) and (11) shows that for the symmetricalcase the dimensioning may be twice that for the unsymmetrical case.

The depth or a: dimension of the main conduit may have any value and inpractice is chosen to permit the passage of the required volume of airthrough the filter. However, if the dimension :1, as shown in Fig. 2,exceeds a wave-length M ii; is advisable to insert one or morepartitions, such as l3, running the full length of the main conduit andpreferably extending into the side branches. These partitions are spacedapart a distance p which does not exceed a wave-length M. In the exampleshown,

x1=2p=2Ao (12) However, it is to be understood that in general iti=hko(13) where h is any. integer, in which case there will be (h-1)partitions required.

The opening into the main branch, such as 8" for the side branch 1,preferably extends for approximately one-third of the width of thebranch and is centrally located. Thus, as shown in Fig. 1, thedimensions e1, 82 and 8a are all equal. If the opening is smaller thanthis the air encounters too high a resistance in flowing into and out ofthe side branch, and if the opening is larger the pressure is notsufficiently uniform over the opening. In either case the efficiency ofthe filter is impaired. The portions, such as ll, of the main conduitforming the openings into the side branches are preferably madecomparatively sharp on the side nearer the discharge end of the filter,as shown in Fig. 2 at 24, and rounded on the side nearer the intake end,as shown at 25.

In order to impede the air flow as little as possible the guides l2, l3and H are preferably streamlined to a certain extent. As shown in Fig. 2the outer end 26 is made rounded and the inner end 21 is comparativelythin. Each guide may be made from a single sheet of metal doubled backupon itself and formed in the shape shown.

If a number ofv pairs of side branches are used, as shown in Fig. 1,each pair is preferably given a different length from the other pairs sothat the various pairs will have different frequencies of resonance, allfalling below In. Fbr example, the branch 1 may have a length 91 and aresonance at f1, branch 6 a length g2 and a resonance at I: and branch alength as and a resonance at is. The frequences f1, f2 and is are chosento provide the maximum sustained attenuation over the frequency rangebelow fa which is to be suppressed. Since the side branches occupy allavailable space along the sides of the main conduit it the distances qr,(12 and Q3 which the branches 1, 8 and 5, respectively, extend along themain conduit 1 determine the lowest frequency which will be suppressedby the filter. The longer these dimensions are made the lower will bethe lower cut-off of the filter. The branches 5', 0' and I are similar,respectively, to the branches I, 6 and l. The principles to be followedin designing the side branches are more fully discussed in Patent1,692,317 to G. W. Stewart issued November 20,

1928, and in my prior Patent 1,874,326 issued August 30, 1932.

The frequencies above f0 will not be attenuated by the sections of thefilter which include the side branches but they will be attenuated to aconsiderable extent in the extension 18 of the main conduit which isdivided by the partition I3 and lined throughout with a layer of soundabsorbing material H as already mentioned. The longer the dimension .9is made the greater will, be the suppression of the frequencies aboveIn.

As a specific example suitable dimensioning for a filter meet a certainset of requirements will be given. It is assumed that the filter is tocarry 3430 cubic feet of air per minute and that the principalattenuation band is to extend from 200 to 1760 cycles per second. Thevelocity of sound in air is taken as 1100 feet per second. Thewidthdimension w of the main conduit 4 is equal to one wave-length atthe frequency 10:1760, and from expression (11) is found to be The depthdimension m1 of the main conduit is chosen as two wave-lengths andtherefore x1=27w=15 inches The spacing p between the central partitionI! and each of the side walls of the main conduit 4 is equal to awave-length and therefore =M=7.5 inches In accordance with expression (8the guides ll,

13 and H in section II have a half wave-length spacing, and therefored=='/Xo==3.75 inches The frequencies of resonance for the side branches1, 0 and I are chosen, respectively, as

f1=250 cycles per second f2=360 cycles per second fs=455 cycles persecond giving the following lengths for the side branches:

of: 10 inches 9r= 6 inches 0:: 4 inches Since the lowest frequency to beattenuated is 200 cycles per second the following are suitablelongitudinal dimensions for the side branches:

q1=18 inches (12:9 inches 13: 6 inches The side branch 1' has an opening8' equal to one-third of its longitudinal dimension qi and thereforee1==e2=es= /3q1=6 inches A suitable length r for the high frequencysection it of the main conduit is 42 inches.

Although the sections of the filter which include the side branches aredesigned to provide an attenuation band extending only from 200 to 17cccycles per second, the high frequency section l8 will furnish enoughattenuation at frequencies above 1760 cycles so that all frequenciesbetween 200 and 8000 cycles are effectively suppressed by the filterstructure as a whole.

If a filter with greater air carrying capacity is required it is onlynecessary to extend the depth dimension :n in increments equal to p andto provide the required numberof partitions such as H with a spacing notexceeding 1). It is to be understood that, in practice, any one or allof the dimensions may vary somewhat from those given. Also, if adifferent frequency band is to be suppressed, the dimensions should bechanged accordingly. The dimensions given for m, p and d are the maximumpermissible for the conditions assumed, and provide the most eflicientuse of material. These dimensions may, of course, be smaller and thefilter will still function to suppress the required band, but the aircapacity will be correspondingly reduced.

-Iibat is claimed is: I

1. An acoustic wave filter for suppressing a bond frequencies comprisinga main conduit of rectangular crosssection and a pair of cppositelydisposed side branches connected to said main conduit, said sidebranches having a fre quency of resonance which falls within the band01'' frequencies to be suppressed, the depth 01' said main conduit beinggreater than X0, said main conduit being divided by longitudinalpartitions parallel the width dimension and the spacing of saidpartitions not exceeding X0.

2. An acoustic wave filter for suppressing a f equencies comprising amain conduit ular cross section and a pair of op pcsitely csed sidebranches connected to said main conduit, said side branches having afrequency of resonance which falls within the band of frequencies to besuppressed, said main conduit having a curved section which includestherein guide partitions disposed parallel to the curved sides of saidsection and the spacing of said partitions not exceeding an.

3. An acoustic wave filter for suppressing a band of frequenciescomprising a main conduit oi rectangular cross-section and a pair ofoppositely disposed side branches connected to said main conduit, saidside branches having a frequency of resonance which falls within theband of frequencies to be suppressed, and a portion of said main conduitbeing lined with a layer of sound absorbing material.

4. An acoustic wave filter for suppressing a band of frequenciescomprising a main conduit of rectangular cross-section and a pair ofoppositely disposed side branches connected to said main conduit, saidside branches having a fre-' of resonance which falls within the band w.to be suppressed, and each of said nunicating with said main the area orwhich is third of the crosson, a pair of oppositely onnected to saidmain partitions extending rallel to the width 1 of said main con- :ipartitions exceeding ending to the highest in accordance with claim 5 inwhich said partitions extend into and divide said side branches.

7. An acoustic wave filter in accordance with claim 5 in which said mainconduit has a curved section and guide partitions disposed parallel tothe curved sides of said section are inserted therein, the spacing ofsaid partitions not exceed ng M.

8. An acoustic wave filter in accordance with claim 5 in which a portionof said main conduit is lined with a layer of sound absorbing material.

9. An acoustic wave filter in accordance with claim 5 in which each ofsaid side branches communicates with said main conduit through anopening the area oi which is equal to approximately one-third of thecross-sectional area 0! the side branch.

10. An acoustic wave filter in accordance with claim 5 which includes asecond pair of oppositely disposed side branches connected to said mainconduit, said two pairs of branches having frequencies of resonancewhich differ from each other but fall within the band of frequencies tobe suppressed.

11. An acoustic wave filter ior suppressing a band of frequenciescomprising a main conduit of rectangular cross-section, a plurality ofpairs of oppositely disposed side branches connected to said mainconduit, longitudinal partitions extending across said main conduit andinto said side branches, neither the width of said main conduit nor thespacing of said partitions exceeding the wave-length in corresponding tothe highest frequency to be suppressed, said pairs of branches havingfrequencies of resonance which are all different but all of which fallwithin the band of frequencies to be suppressed, and a portion of saidmain conduit being lined with a layer or sound absorbing material.

12. An acoustic wave filter in accordance with claim 11 in which saidmain conduit has a curved section and guide partitions disposed parallelto the curved sides of said section are inserted therein, the spacing ofsaid partitions not exneeding lo.

13. An acoustic wave filter in accordance with claim 11 in which each ofsaid side branches co'mmunicates with said main conduit through anopening the area of which is equal to approximately one-third of thecross-sectional area of the side branch.

14. An acoustic wave filter in accordance with claim 11 in which saidmain conduit at one end gradually flares to a larger cross-sectionalarea.

15. An acoustic filter for suppressing a band of frequencies comprisinga main conduit of rectangular cross-section and a pair of oppositelydisposed side branches connected to said main conduit, the width of saidmain conduit being approximately equal to the wave-length incorresponding to the highest frequency to be suppressed.

16. An acoustic wave filter in accordance with claim 15 in which thedepth of said main conduit is greater than M and said main conduit isdivided by longitudinal partitions parallel to the width dimension, thespacing of said partitions being approximately equal to 17. An acousticfilter in accordance with claim 15 in which the depth oi said mainconduit is equal approximately to 1m, where h is any integer and saidmain conduit is divided by (In-1) equally spaced longitudinal partitionsdisposed parallel to the width dimension,

18. An acoustic wave filter for suppressing a band of frequenciescomprising a main conduit of rectangular cross-section having a straightsection and a curved section, a pair of oppositely disposed sidebranches connected to said main conduit and longitudinal partitionsextending across said main conduit parallel to the width dimension, thewidth of said main conduit being approximately equal to the wave-lengthM corresponding to the highest frequency to be suppressed, the spacingof said partitions in said straight section being equal approximately toM and the spacing of said partitions in said 5 curved section beingequal approximately to WARREN P. MASON.

